1. Field of the Invention
This invention generally relates to a stacked solid electrolytic capacitor.
2. Description of the Related Art
Recently, there are demands for enhancing capacity of an electrolytic capacitor, for downsizing the electrolytic capacitor and further for lowering impedance at higher frequencies of the electrolytic capacitor, as electronics devices are digitalized. The solid electrolytic capacitor has an excellent frequency characteristic relative to other electrolytic capacitors, and thus attracts attentions. A roll-formed solid electrolytic capacitor, a stacked solid electrolytic capacitor and so on are used as the solid electrolytic capacitor.
A conventional electrolytic capacitor has an aluminum foil acting as a valve metal, and has an electrolyte layer composed of polypyrrole formed by a chemical polymerizing method, an electropolymerizing method or the like. The electrolytic layer composed of the polypyrrole is, however, not evenly formed on the surface of the aluminum foil, and is brittle. The stacked electrolytic capacitor has, therefore, problems such as increase of a leakage current or an electrical short.
And so, Japanese Patent Application Publication No. 2002-299183 (hereinafter referred to as Document 1) discloses a method where a precoated layer composed of manganese dioxide is formed in advance and the polypyrrole is formed on the precoated layer later. However, the arrangement disclosed in Document 1 has a problem that equivalent series resistance (ESR) increases, because there is a resistance between the precoated layer and the electrolyte layer. It is, therefore, not possible to obtain sufficient property.
And so, Japanese Patent Application Publication No. 2002-75792 (hereinafter referred to as Document 2) discloses an art where a separator is provided between a cathode foil and an anode foil, and a polyethylene terephthalate (PET) formed by a heat and pressure polymerization is used as the electrolyte layer. It is possible to enhance capacity, if the solid electrolytic capacitors in accordance with the art of Document 2 are stacked. And Japanese Patent Application Publication No. 2005-39040 (hereinafter referred to as Document 3) and Japanese Patent Application Publication No. 2005-72106 (hereinafter referred to as Document 4) discloses an art where a cathode layer composed of a graphite layer and a silver paste layer is provided. The arrangements disclosed in Document 3 and Document 4 can solve a problem concerning a high-frequency response.
However, there is a measurable connection resistance in the solid electrolytic capacitor in accordance with the art of Document 2, because the electrodes and outgoing leads are connected electrically. And it is necessary to connect the outgoing leads as many as the stacked electrode foils, although it is possible to enhance capacity by stacking the electrode foils. And the connection resistance increases. Accordingly, the high frequency response gets worse. And it is difficult to reduce the thickness of the electrolytic capacitor as the quantity of electrode foils increases.
Further, in the solid electrolyte capacitor in accordance with the arts of Document 3 and Document 4, a short circuit may be generated at the graphite layer and the silver layer, because there is a fault at a dielectric oxide layer of the anode.